Android: Smart WiFi Toggler is a free utility that automatically learns where your preferred wireless networks are located, and when you approach their location, will turn on wi-fi and join those networks. When you leave the network's range, the app will turn off your phone's wi-fi radio so you don't waste battery searching for networks while you move around.

If you frequently switch between wi-fi at home or the office and 3G/4G on the go, you know it sucks to look down at your phone only to see you left wi-fi on while you're driving and your battery has suffered as a result. XDA forum member sebouh00 designed Smart WiFi Toggler to handle this problem, and once installed, the app does most of the work for you.

The app uses your phone's network location so you don't trade one battery drain (scanning for wi-fi networks) for another (scanning for GPS location), and automatically turns on and joins your preferred wi-fi networks without your intervention. The app even automatically learns where those networks are located so you don't have to program it manually.

Granted, Smart WiFi Toggler doesn't do anything that JuiceDefender or Tasker can'talready do, but Smart WiFi Toggler is completely free, less complicated, and does one thing very well.